برگردان داده‌های میکروگرانی برداشت‌شده در اطراف سد سیاه‌بیشه برای تعیین ساختارهای زیرسطحی در مسیر ساخت یک تونل

نویسندگان

1 کارشناسی ارشد، گروه ژئوفیزیک، واحد همدان، دانشگاه آزاد اسلامی،همدان، ایران

2 دانشیار، گروه فیزیک، دانشکدة علوم دانشگاه اراک، ایران

3 استادیار، گروه ژئوفیزیک، واحد تویسرکان، دانشگاه آزاد اسلامی، تویسرکان، ایران

4 استاد، گروه فیزیک زمین، مؤسسة ژئوفیزیک، دانشگاه تهران، ایران

چکیده

میکروگرانی یک روش ژئوفیزیکی است که به‌طور فزاینده‌ای برای بررسی‌های محیطی و ژئوتکنیکی به کار می‌رود. اندازه‌گیری‌های میکروگرانی به تغییرات چگالی زیرسطحی پاسخ می‌دهد و روشی غیرمخرب برای شناسایی و تشخیص ناهمگونی‌های زیرسطحی است. مواد زیرسطحی گوناگون چگالی‌های جرمی متفاوتی دارند؛ بنابراین برداشت‌های میکروگرانی، با جمع‌آوری اندازه‌گیری‌های سطحی میدان گرانی زمین، نواحی شامل ناهنجاری‌ها یا تباین‌های جرم حجمی را جستجو و کاوش می‌نماید.
هدف از این پژوهش تفسیر داده‌های میکروگرانی برداشت‌شده در قسمت کوچکی از محل احداث سد تلمبه‌ای سیاه‌بیشه در شمال ایران، جهت تعیین کیفیت و نوع ساختار تشکیلات زیرسطحی در محل حفر تونلی است که به محدودۀ ریزشی برخورد کرده است. بعد از پردازش و تصحیحات جزرومد، رانه و هوای‌آزاد و بوگه و توپوگرافی، بی‌هنجاری بوگه محاسبه شده است. با استفاده از برازش چندجمله‌ای‌های متعامد و بهنجار با داده‌های بی‌هنجاری‌های گرانی بوگۀ به‌دست‌آمده، آثار گرانی منطقه‌ای برآورد و سپس بی‌هنجاری‌های باقیمانده محاسبه شدند. سه بی‌هنجاری منفی در نقشۀ گرانی باقیمانده شناسایی شده است. داده‌های گرانی این بی‌هنجاری‌ها با روش معکوس‌سازی سه‌بعدی، با به‌کار بردن نرم‌افزار گروت2 مدل‌سازی شده‌اند. نتایج حاصل از برگردان‌سازی داده‌ها، توزیع تباین چگالی بی‌هنجاری‌های جرمی سه‌بعدی بوده است که از آن‌ها برش‌هایی افقی و مقاطع عمودی، به موازات محورهای سه‌گانه مختصات کارتزین، انتخاب و به تصویر کشیده شده‌اند. سپس این توزیع تباین چگالی‌های جرمی ترسیم شده در مقاطع سه‌گانه با استفاده از اطلاعات میدانی زمین‌شناسی و یک گمانه به ساختارهای مختلف زمین‌شناسی زیرسطحی نسبت داده شده‌اند. در روی مقاطع، نواحی بی‌هنجار با چگالی کم، به تشکیلات آهکی خردشده، هوازده و محتملاً شامل آب نسبت داده شده است. این نواحی به علت نفوذپذیری و تراکم کم از مناطق آسیب‌پذیر و ریزشی در حفر سازۀ تونل تشخیص داده می‌شوند. نواحی شناسایی‌شده با تباین چگالی جرمی بیشتر در روی مقاطع، به وجود سنگ‌های آهکی متراکم یا سایر تشکیلات مقاوم‌تر مثل آذرین نفوذی، با آسیب‌پذیری و خطر ریزش کمتر در ایجاد سازۀ تونل، نسبت داده شده است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Inversion of microgravity data around Siah Bisheh dam, for determination of subsurface structures in a tunnel construction path

نویسندگان [English]

  • Maryam Chegeni 1
  • Mahmoud Mirzaei 2
  • Mojtaba Babaei 3
  • Vahid Ebrahimzadeh Ardestani 4
1 M.Sc. in Geophysics, Department of Geophysics, Hamedan Brench, Islamic Azad University, Hamedan, Iran
2 Associate Professor, Department of Physics, Faculty of science, Arak University, Iran
3 Assistant Professor, Department of Geophysics, Tuyserkan Brench, Islamic Azad University, Tuyserkan, Iran
4 Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran
چکیده [English]

The gravity method is one of the geophysical tools used for geological, engineering and environmental investigations where the detection of geological boundaries, cavities, subsurface karstic features, subsoil irregularities, or landfills are essential. In higher accuracy measurements, the microgravity method has been widely and successfully used for locating and monitoring subsurface materials.
 Since microgravity methods measure gravity variations at the surface, they are directly influenced by the density distribution in the subsurface and particularly by the presence of formation material, which may create a mass deficit relative to the density of the surrounding terrain. In many cases, deep or small-scale heterogeneities generating low-amplitude anomalies can be detected and the reliability of further interpretation requires highly accurate measurements which are carefully corrected for any quantifiable disturbing effects. The main purpose of the research, that was conducted in small part of a dam site, is to determine the quality and type of subsurface structures in location of tunnel construction. Study area for collecting microgravity data was located at a small part, considered for construction of Siah Bisheh dam, road Tehran to Chalous. Position of microgravity stations were over a tunnel path which in some parts encountered with collapsing structures. The study area was part of Alborz Mountains. Geology formation(Shemshak formation), consisting of lime beds together with igneous rocks which are severely affected by fractures. Data were collected along 13 profiles with separating distance of 15 m. The stations distance and number of data were 15 m and 148 respectively. Bouguer gravity anomaly was calculated after making corrections such as earth tide, free air, Bouguer, topography and terrain effects. The regional effect obtained using a program that is written in FORTRAN to fit orthogonal and orthonormal polynomials on the observed data and then residuals were estimated. Three negative anomalies were distinguishable in residual gravity map. Data of these anomalies are modeled with a 3-D inversion approach using GROWTH 2.0 software. The GROWTH 2.0 is an inversion tool which enables the user to obtain, in a nearly automatic and non-subjective mode, a 3D model of the subsurface density anomalies based on the observed gravity anomaly data. The current version of the tool has been developed from an earlier code (Camacho et al., 2002). In a nearly automatic approach, the software provides a 3-D model informing on the location and shape of the main structural building blocks of the subsurface structures. Then densities contrast of these anomalies was estimated. Result of the inversion was a 3-D distribution of densities contrast. To show this distribution of the densities contrast, the horizontal and vertical sections at different depth and different horizontal positions were selected and interpreted. From these sections it is indicated that the effective depth of the data, for identifying martial of subsurface structures from the inversion, is about 50 m. In the sections, areas with low densitig contrasts are related to the fractured limestone and those with high contrast ones are related to the compact limestone or igneous rocks. Existence of igneous and lime rocks that have more density and compactness, increase the quality of the structures in the path of the tunnel construction. Areas including fractured limestone, with lower density, decrease the quality of the structure and increase the risk of water permeability and collapsing in the path of the tunnel construction. Thus by interpreting of the results of the microgravity data inversion, areas with high and low compactness and good and bad quality rocks for tunnel construction are recognized, those are related to the fractured or karstic limestone and limestone and igneous rocks. Also boundaries of these formations where densitig contrasts vary suddenly, are related to the existence of faults.

کلیدواژه‌ها [English]

  • Microgravity
  • gravity anomaly
  • 3-D inversion
  • Contrast density
  • tunnel
  • compactness
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